Fluoropolymers are applied to a wide number of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. Coatings of polytetrafluoroethylene (PTFE) homopolymers and modified PTFE provide the highest heat stability among the fluoropolymers, but unlike tetrafluoroethylene (TFE) copolymers, cannot be melt processed to form films and coatings. Therefore, other processes have been developed for applying coatings of PTFE homopolymers and modified PTFE. One such process is dispersion coating which applies the fluoropolymer in dispersion form. Dispersion coating processes typically employ such fluoropolymer dispersions in a more concentrated form than the as-polymerized dispersion. These concentrated dispersions contain a significant quantity of nonionic surfactant, e.g. 6-8 weight percent. As disclosed in U.S. Pat. No. 6,153,688 to Miura et al. and U.S. 2003/0130393 to Cavanaugh et al., it is desirable to use aliphatic alcohol ethoxylate nonionic surfactants to avoid environmental concerns associated with aromatic group-containing nonionic surfactants, e.g., alkyl phenol ethoxylates.
Dispersion coating processes include the steps of applying concentrated dispersion to a substrate by common techniques such as spraying, roller, curtain coating, or dip coating; drying the substrate to remove volatile components; and baking the substrate. When baking temperatures are high enough, the primary dispersion particles fuse and become a coherent mass. Baking at high temperatures to fuse the particles is often referred to as sintering.
For a number of dispersion coating applications such as curtain coating or seriography, a fraction of the coating stream is deposited on the substrate requiring the remainder of the stream to be recycled. The recycled fraction needs to be able to withstand the subsequent multiple pumping and mixing operations necessary for a continuous process. A dispersion suitable for such processing should not easily coagulate when subjected to shearing forces. The resistance of the dispersion to premature coagulation can be measured by a parameter known as gel time and is an indication of the shear stability of the dispersion.
It has been recognized in U.S. Pat. No. 6,841,594 B2 and US2003/0008944 A1 to Jones et. al. and US2003/0130393 A1 to Cavanaugh et al. that certain non-melt processible fluoropolymers of a core/shell configuration having a core of high molecular weight PTFE and a shell of lower molecular weight PTFE or modified PTFE possess excellent shear stability. Fluorosurfactants are typically used as an ingredient in the dispersion polymerization of these fluoropolymers, the fluorosurfactants functioning as a non-telogenic dispersing agent. For example, an early description of this use of fluorosurfactants is found in U.S. Pat. No. 2,559,752 to Berry. However because of environmental concerns and because fluorosurfactants are expensive, processes have been developed for their recovery from waste water and from aqueous fluoropolymer dispersions.
There are several known techniques for the removal of fluorosurfactants from fluoropolymer dispersions. One method is disclosed in U.S. Pat. No. 4,369,266 to Kuhis et al. and includes the addition of a stabilizing surfactant followed by concentration by ultrafiltration. This patent teaches that a high proportion of the fluorosurfactant can be removed via the aqueous permeate. It is also known to remove fluorosurfactant by adsorption onto an ion exchange resin as taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls). Kuhis teaches recovery of fluorinated emulsifiers dissolved in the aqueous phase after coagulation of the polymer from the dispersion or in aqueous polymer dispersions to be concentrated. US 2003/0125421 A1 (Bladel et al.) also teaches removal of fluorine-containing emulsifiers from fluoropolymer dispersion by contacting with an anion exchanger.
In fluoropolymer dispersions with low fluorosurfactant content, the dispersion tends to undergo an unacceptable viscosity increase after fluorosurfactant removal or upon concentration making the dispersions unsuitable for room temperature coating applications. U.S. Pat. No. 6,861,466 B2 to Dadalas et. al. discloses the viscosity of aqueous fluoropolymer dispersions with low fluorosurfactant content may be controlled by the addition of anionic non-fluorinated surfactant. However, the presence of anionic non-fluorinated surfactant in fluoropolymer dispersions is undesirable for some applications, e.g., dispersion coating of glass cloth where undesirable color can be imparted by anionic non-fluorinated surfactant. In metal coating applications, anionic non-fluorinated surfactants can restrict the range of possible coating formulations, i.e., limit formulation flexibility in such applications.
A further problem fluoropolymer dispersion with low fluorosurfactant content is that the Critical Cracking Thickness (CCT) in coating applications is reduced unless agents such as acrylic binders are added. CCT is a measure of the thickness of a coating formed from polymer dispersion that can be applied to a substrate in one pass without cracking during drying and subsequent baking.
Improved fluoropolymer dispersions with low surfactant content are needed.